Seismic surveying



May 14, 1946.. W, H MAYNE 2,400,245

n SEISMIC SURVEYING Filed Jan. 21, 1944 l 76: 75 /ff @15M-y H/ ierafderPatented May 14A, 1946 2,400,245 sErsMlc sURvEYrNG William Harry Mayne,San Antonio, Tex.. as-

signor to Olive S. Petty,- San Antonio 6, Tex.

Application-January 21, 1944, Serial No. 519,180 3 Claims. (Cl.177-35'2l This invention relates to apparatus for use in seismicsurveying and has for its principal object the provision of an improvedand simplifiedl form of control for such apparatus, especially adaptedfor use in controlling and communicating with one or more amplifierunits from a remotely disposed recording unit.

Apparatus commonly used in the field of seismic surveying includes asits essential elements the detector or seismometer, the amplifier, andthe recorder. A seismic disturbance is initiated by the firing of acharge of explosive at or near the surface of the earth, therebygenerating and propagating seismic impulses or waves through the earth.These waves are then picked up at spaced remote points and convertedintowaveform electrical energy by seismometers. The electrical energyoutput of each seismometer is then amplied and recorded.

In certain types of seismic worky for instance refraction survey'ing,the seismometers are spaced at considerable distances and it istherefore pref-l erable, in order to reduce the noise level as much aspossible, -to locate each amplifier unit near the detector from which itis supplied with energy. The outputvof each amplifier is thentransmitted to a common'recording station by a two-conductor cable. Itis for use with such a system that the present invention is especiallyintended since it provides for the control of and communication with thevarious detector stations from the recording station without requiringthe use of additional conductors or cables between these stations, thusreducing the cost of the system and simplifying its maintenance andoperation.

More specifically, it is an object of the. invention to provide, in aseismic system in which the amplifier and recording stations are remotecommunication means and control means operating over the cables whichdeliver energy from the amplifiers to the recording station. Preferablythe control means is of such a nature that from the recording stationthe amplifiers can be turned on and off, the polarity of the cableconductors checked, the, amplifier batteries tested, and the gain of theamplifier controlled.

Further objects and features of the invention will be apparent from thefollowing description taken in connection with the accompanying draw'conducting refraction surveys, for which itis particularly suited, butnol limitation of the invention is intended thereby. V For the purposeof simplifying the drawing and descriptive matter, one complete unit isshown, it being understood that the usual system involves the employmentof a plu-` rality of seismometers disposed in spaced relation,` andarranged to function simultaneously to convert the mechanical energy ofthe seismic irnpulses into electrical energy, the output'of eachseismometer being separately amplified, and the amplified energy beingdelivered to a suitable multiple recorder.

In the drawing the seismometer is shown at I0 and the amplifier for theseismometer at I2. 'I'he amplifier is of the usual thermionicfvalvetype, slightly modified to provide the advantages of the instantinvention.

fThus the leads from the seismometer are connected respectively to thegrid and cathode of a thermionic valve I3, The output of valve I3 is fedthrough a resistance coupled stage to the grid of a second valve I6, anadjustable contact with the grid resistor I 5 being provided in orderthat the maximum desired gain of the amplifier may thereby bedetermined. The resistance coupled stage is slightly modified forfiltering purposes by the use of a condenser 2li in shunt with the anoderesistor, which serves to by-pass undesirable high frequencies,.and bythe use of condensers 2| and 22 which function as hereinafter more fullyexplained. Y

The output of valve4 I6 is delivered through second resistance coupledstage to valve I8, a condenser Il, functioning similarly to condenser 20to reduce undesirable high frequencies, being l shunted across the anoderesistor. Valves I3 and I6 are supplied with anode voltage from a source24, which is also used as the source of voltage of a high potential gridin valve I8; the source of anode voltage of valve I 8 is indicated at26.

It will be appreciated that the structure so far described isconventional, and that the details thereof form no essential part of theinstant ini vention but may -be widely modified within the limits ofappropriate amplifier design.

The anode current from valve I8 is fed through resistor 29 and throughthe winding of a relay 30 to the cathodes of the several valves.Fluctuations in energy supplied to the amplifier cause a pulsatingcurrent to flow from the anode of valve I8 through primary winding 3B oftelephone transformer 31 into one conductor of the two-conductor outputcable 4|) which extends from the amplifier to the recording station; the

2 length of this cable may be of the order of from 1500 to 5000 feet. Atthe recording station, the

' ductor of the cable 40 to the valve cathodes.

Condensers 5I and 52 are arranged in shunt in the circuit which isdescribed, the `impedance of these condensers being relatively high sothat they introduce little loss so far vas the low frequency wave formenergy is concerned, but freely pass voice current flowing in thecircuit through the primary windings of the telephone transformers 31and 44. These windings, 36 and 43, are so designed that they offer verylow impedance to the low frequency seismic energy. The secondary winding55 of telephone transformer 31 is connected to a telephone set indicatedgenerally at 56, a, series condenser 58 being employed to impede theflow in this circuit of frequencies of the order of seismic frequencies.The secondary winding 60 of telephone transformer 44 is connectedthrough a transformer 6I to a second telephone set 62 at the recordingstation, andthe winding 60 may be connected in parallel with thewindings of transformers similar to transformer 44 which are similarlyconnected with the twoconductor cables running to the recording stationfrom other amplifiers. The telephone sets 56 and 62 may be of the voiceactuated type, the showing in the drawing being diagrammatic. A switch61 in shunt with the primary winding of transformer 6I may be closed tointerrupt telephonic communication.

The secondary Winding 64 of output transformer 49 is connected toenergize a, moving element in the multiple recorder'66, other movingelements of this recorder being similarly fed with wave form energyreceived from other amplifiers.

It will be appreciated that by the use of the circuits just described,voice communication may be held between each amplifier and the recordingstation through the output cable associated with that amplifier, theconstruction being such that seismic energy and voice signals arenot-affected one by the other but are effectively segregated by theseveral filters employed. It is found that with the use of thisarrangement and without substantial added expense or increasedydifilculty in handling the cables, the speed of field operations can betremendously increased, since it is frequently necessary to establishcommunication between the operator at the recording station andoperators stationed at the variouspoints of de-` tection andamplification of the seismic impulses. This feature of the invention isespecially valuable in refraction work where distances between theparties may be of the order of 5000 feet or more.

In order to reduce the cost of cables and to facilitate the handling ofthe equipment, it is helpful in refraction surveying to locate theamplifier batteries orvoltage sources at the amplier, but frequentreplacement of these batteries is necessary unless care is exercised todisconnect them whenv the amplifiers are not in actual use for theamplification of seismic energy. It isA therefore a feature of theinvention that any of the amplifiers may be turned on or off from therecording station. It is also desirable to effect scribed by the use ofthe same output cable 40,

The heating or cathode current for the ampli,-

fier valves is provided by a source of voltage 68 which is connected tothe valves through a switch CII switch 1,6 is closed, current ows in thewinding 36 and the switch 69 is thereby closed to heat the cathodes ofthe valves. When the cathodes are heated to a temperature sufcientlyhigh to cause flow of anode current in the valves, the anode currentfrom valve I8 ows through the winding 36 and thus maintains the switch69 closed, so that switch 16 may thereafter be opened withoutinterrupting the operation of the amplifier. If it is desired todiscontinue operation of the amplifier, switch 18 may be closed to shortcircuit resistor 29 and relay winding 30, and on cooling of the cathodesand resultant interruption of anode current, the switch 16 may beopened, leaving the amplier turned off.

Similarly, the gain of the amplifier can be controlled from therecording station by closing switch 16 and varying resistor 15, tothereby vary the voltage applied by the source 12 across resistor 29 andwinding 30. VSuitable increments of the voltage thus applied are thenreturned through leads 8l and 82 and resistors 94 and 85 to the grids ofvalves I3 4and I6. of source -12 is preferably such that on reduction ofthe effective value of the resistor 15,v these grids are rendered morenegative so that the gain of the amplifier may be suitably reduced fromthe maximum gain determined by the setting of the variable contact fromthe grid of valve I6 to resistor I5. Condensers 2| and 22 function toby-pass any audio components of the output appearing across resistor 29and winding 30, thus effectively blocking from the amplifier the seismicand voice frequencies transmitted through cable 40 while permitting thefeeding to the amplifier of the gain control voltage established at therecording station. Valves I3 and I6 are preferably of the variable Mutype, valve I3 having a relatively close cutoff and valve I6 having aremote cutoff. Thus by returning the gridsof these valves to properpoints in the circuit through the leads 8| and 82, it is possible toobtain the desired ywhile maintaining appreciable current flow in thecircuit. The output of the pentode tube, when so used, can be controlledin any conventional manner, for example, by control of grid bias.

The two-conductor cable 40 may be replaced by .a single conductor,return being effected through the ground. In such event, the ground isconsidered the second conductor for the purposes of the presentinvention.l

It will be perceived from the foregoing that the invention contemplates,among other features, the provision of means affording telephoniecommunication and amplifier gain' control by way of the output cablewhich feeds the amplified seismic signal to the recorder, in combinationwith means blocking from the amplifier both the seismic and the voicefrequencies flowing in the output cable, so that only the gain controlvoltage established The polarity i 'a,4oo,a45

at the recording station is fed back into the ampler. l

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is: l

- 1. In apparatus for use in seismic surveying,-

seismic frequencies on said elements and of voice frequencies on bsaidrecorder, gain control means associated with said amplifier, said gaincontrol means including connections to the valve grids for applyingthereto a variable bias voltage, means at the recording stationoperatively connected through said `conductors with said gain controlmeans to provide vthe said bias voltage and thereby to effect regulationfrom said recording station of the amplifier gain, and filter meansassociated with said gain control means for blocking from the amplierthe energy of voice and seismic frequencies transmitted through saidconductors while permitting the application to said grids of the biasvoltage provided at the recording station.

2.'In apparatus for use in seismic surveying, the combination with aseismometer for converting seismic impulses to electrical energy, anamplier for such energy, and a recorder at a remotely located recordingstation for recording' the amplied energy,l of two conductors aording apath over which the amplified energy is voice frequency transmitting andreceiving elements located adjacent the amplifier and the recordingstation and operatively connected through said conductors, meansassociated with said conductors for inhibiting the eect of seismicfrequencies on said elements and of voice frequencies on said recorder,gain control means associated with said amplier, means at said recordingstation operatively connected ,through said conductors with said gaincontrol means to eiect regulation from said recording station of theamplifierl gain, and filter means associated with said gain controlmeans for blocking from the ampliier the energy of voice and seismicfrequenciestransmittedthrough said conductors.

3. In apparatus for use in seismic surveying,

ithe combination with a seismometer for converting seismic impulses toelectrical energy, a thermionic valve amplifier for such energy, and arecorder at a'remotely located recording station for recording theamplified energy, of two conductors affording a path over which theamplified venergy is transmitted from the amplifier to the recorder,gain control means associated with said amplier, said gain control meansincluding connections tothe valve grids for applying thereto throughsaid conductors while permitting-the applicationto said grids of thebias voltage yprovided transmitted froml the amplifier to the recorder,

